Macrocyclic polyester oligomers (MPOs) have unique properties that make them attractive as matrix-forming resins for engineering thermoplastic composites. MPOs lend valuable characteristics to polymerized products, for example, high strength, high gloss, and solvent resistance. Furthermore, because certain MPOs melt and polymerize at temperatures well below the melting point of the resulting polymer, polymerization and crystallization can occur virtually isothermally upon melting of the MPO in the presence of an appropriate catalyst. The time and expense required to thermally cycle a tool is favorably reduced, because demolding can take place immediately following polymerization, without first cooling the mold.
Various methods for preparing MPO by depolymerizing polyesters have been described. For example, polybutylene terephthalate (PBT) and other polyalkylene terephthalates may be depolymerized to form macrocyclic polyester oligomers (MPOs), including, for example, the cyclic form of poly(1,4-butylene terephthalate) (cPBT). See, e.g., co-owned U.S. Pat. No. 5,039,783 by Brunelle et al., U.S. Pat. No. 5,231,161 by Brunelle et al., U.S. Pat. No. 5,407,984 by Brunelle et al., U.S. Pat. No. 5,668,186 by Brunelle et al., U.S. Pat. No. 6,525,164, by Faler, U.S. Pat. No. 6,787,632 by Phelps et al.; U.S. Pat. No. 7,732,557 by Phelps et al.; U.S. Pat. No. 7,750,109 by Phelps et al.; and U.S. Pat. No. 7,767,781 Phelps et al.; the texts of which are all incorporated by reference herein in their entirety.
The depolymerization reaction is an equilibrium reaction that progresses relatively slowly and produces undesired byproducts, including hydroxybutylester linear oligomers (referred to herein as “linears”), which must be separated from the product stream, or recycled. These byproducts are typically gellular in nature, and are physically difficult to filter or otherwise remove from solution. Furthermore, the above depolymerization methods require precipitation and removal of catalyst residue from the reaction solution. The separation, extraction, and/or recycle of linears and/or catalyst residue necessitate added process steps and unit operations in the manufacture of MPOs, thereby increasing both capital expense and operating costs.
Thus, there is a need for a faster, more efficient, less costly method of manufacturing MPO.